The proposed study will investigate the function of quiescent leukemic cells in non B - non T acute lymphoblastic leukemia in children. In multivariate analysis of prognostic factors, and in sequential studies in patients, quiescent cells with low cellular RNA content have been identified as resistant to early induction therapy. They may also constitute the source of leukemic cells at relapse. Cellular DNA content, RNA content and the expression of specific proteins involved in cell cycle regulation (s30 "statin", PCNA, p53, histone HI-0, transferrin receptor) will be studied by new flow cytometric techniques and their interactions will be investigated. Quiescent cells will be separated by fluorescence activated cell sortin for further morphological and immunological evaluations. Recent data indicate the complete differentiation of a small subpopulation of leukemic "null" cells to mature B cells in untreated patients. These cells are quiescent and preliminary studies indicate their resistance to chemotherapy. Multiparameter flow cytometric analysis of cell surface immunoglobulin light chain expression, DNA, and RNA content will elucidate the relationship between subgroups of quiescent and differentiated leukemic cells. Sorted subpopulations with differences in RNA content and immunoglobulin expression will be analyzed for heavy and light chain gene rearrangements and transcription of the rearranged genes. A model will be developed to better define the prognostic importance of quiescent/differentiated cells in pediatric ALL. In vitro studies will attempt to recruit quiescent cells into the cell cycle and render them more sensitive to induction therapy. If the studies are successful, patients at increased risk of relapse will be identified at diagnosis and new therapeutic modalities could be developed.